Memoir 5—The Mississippian of west-central New Mexico

By A. K. Armstrong, 1958, 49 pp., 4 tables, 21 figs., 6 plates, 1 index.

Mississippian rocks were studied in detail in the Ladron, Lemitar, and Magdalena Mountains, and Coyote Hills, of west-central New Mexico. The oldest Mississippian strata in this area belong to the Caloso Formation, which contains a small brachiopod fauna suggestive of a Kinderhookian or lower Osagian age. The Caloso Formation rests on a truncated igneous and metamorphic complex of Precambrian age. The base of the Caloso Formation consists of intergrading arkoses, sandstone, shales, and limestones. This is followed by about 30 ft of dark-gray fine-crystalline algal limestone. The Caloso Formation is overlain disconformably by the Kelly Formation, which varies from 0-70 ft in thickness and is light-tan to gray crinoidal limestone that contains a brachiopod and blastoid fauna of upper Osagian age. The Kelly Formation is overlain unconformable by the Sandia Formation of Pennsylvanian age in the Lemitar and Magdalena Mountains. In the Ladron Mountains the Kelly and Caloso Formations pinch out northward beneath the Sandia unconformity. The brachiopod, blastoid, and coral fauna of the Kelly and Caloso Formations are described and illustrated. This paper presents the results of faunal and stratigraphic studies of the Mississippian of west-central New Mexico. Exposures are limited largely to the Magdalena, Lemitar, and Ladron Mountains. From a regional viewpoint, the Mississippian strata of this region fill a gap between those of southern and those of northern New Mexico. The region covered in the present report is indicated. From a stratigraphic viewpoint, the Mississippian of west-central New Mexico has proved somewhat anomalous in relation to either southern or northern New Mexico. The two formations in the region under discussion, the Caloso and Kelly Formations, are not typically developed in any section containing either the Arroyo Peñasco Formation of the north of the Caballero and Lake Valley Formations of the south. This report is concerned almost exclusively with the Mississippian strata present in the Magdalena, Lemitar, and Ladron Mountains. The Magdalena Mountains are a northward trending range with a maximum elevation of over 10,000 ft. They consist of tilted and faulted sedimentary and volcanic rocks on a Precambrian basement. The stratified rocks form the western slope of the range. The eastern side of the range consists mainly of Precambrian granite.